Method of focusing electron microscopes



Patented Sept. 16, 1941 METHOD OF FOCUSING ELECTRON MICRO- SCOPES HelmutRuska, Berlin-Nikolassee, Germany, as-

signor to Fides Gesellschaft fiir die Verwaltung und Verwertung vongewerblichen Schlitzrechten mit beschr'ankter Haftung, Berlin, Germany,a corpcration of Germany No Drawing. Application February 28, 1940, Se-

rial No. 321,237. In Germany September 16,

Claims.

The present invention relates to a method of improving or facilitatingthe focusing of electron microscopes.

In the operation of electron microscopes, especially for photographicreproduction, it is ve y important that the image of a substance to beexamined in the microscope is focussed sharply on the fluorescentscreen. However, there are numerous substances, for example, viri whichhave very little contrast. When adjusting the electron microscope, it istherefore very difflcult to show the image of such substances in focuson the fluorescent screen and to determine whether the image is sharplyadjusted or out of focus. This difflculty is enhanced by the fact thatthe intensity of the electron beam decreases in proportion to theincrease of the magnification factor, so that with high magnificationsthe fluorescence of the screen is only very low and wgak contrasts inthe image are hardly percepti le.

It is the principal object of the present invention to overcome theabove mentioned disadvantages according to the invention, the substancesto be examined are provided with means contrasting therewith, so thatthe dinerence in contrast between the added means and the actual imageof the substances can be easily determined on the fluorescent screen.The strong contrasts thus appearing in the magnified image of theconglomerate permit an easy and sharp focusing of the image.

In practicing the invention, the substances to be examined in theelectron microscope, for example viri, are provided with materialshaving a specific density considerably higher than the density of thesubstances. Only small particles of these materials are added to thesubstances to be examined, and the size of these particles mustcorrespond substantially to the size of the substances. The addedmaterials must be of a type suitable for obtaining in the image on thefluorescent screen of the electron microscope the strong contrastsnecessary for adjusting the image to be in sharp focus. It is advisableto use only such materials which do not react in any way with thesubstance itself, and it is easily possible, for example by tests, todetermine the type of materials best suitable for a particular purpose.For examining viri, colloids of various metals, for example gold orsilver colloids, have been found suitable, although colloidal metallicoxides may also be used.

The following is a more specific example of the method above described.The substance, in

particular virus, to be examined is dissolved and gold colloid is addedto the solution. The concentration of viri, for example, may be in theamount of 10-- to 10* grams of virous protein for each cubic centimeterof solvent. The concentration in gold to be added thereto may then, forexample, amount to 10- to 10- grams per cubic centimeter. The solvent tobe used should be a volatile substance, for example, water or xylenewithout any contents in nonvolatile substances or salts. Thegold-containing solution is applied to a supporting foil as customary inelectron microscopes for supporting the object to be studied. Aftervolatilization of the solvent,

the dried residue left on the foil is a conglomerate of the substance tobe studied and gold particles. The foil is then placed into the electronmicroscope and the focusing is effected in the customary manner, exceptthat the strong contrasts now appearing in the magnified image renderthe focusing very easy and accurate.

I claim:

1. The method of electron-microscopically examining organic substancesof weak electronoptical contrasts, which comprises adding to the organicsubstance to be examined colloidal particles of an inorganic materialchemically inert With respect to said organic substance and of greaterdensity than said organic substance so as to form a conglomerate ofstrong electronoptical contrasts, subjecting the conglomerate thusobtained to electron-microscopical magnification, and adjusting themagnification by focusing the magnified image on the contrasts of theconglomerate.

2. The method of electron-microscopically examining substances of Weakelectron-optical contrasts, which comprises adding to the substance tobe examined a colloid non-reactive as regards said substance and ofgreater specific weight than said substance so as to obtain aconglomerate with strong electron-optical contrasts, subjecting saidconglomerate to electronmicroscopical magnification, and adjusting themagnification by focusing the magnified image on the contrasts of theconglomerate.

3. The method of electron-microscopically examining organic substancecontaining virus or other organisms of Weak electron-optical contrasts,which comprises admixing to said organic substance an inorganic materialnon-reactive as regards said substance and of considerably greaterdensity than said substance, said material being finely subdivided intoparticles of a size in the order of magnitude of the size of saidorganisms,' whereby a conglomerate of strong electron-optical contrastsis obtained, subjecting said conglomerate to electron-microscopicalmagnification, and adjusting the magnification by focusing the magnifiedimage on the contrasts of the conglomerate.

4. The method of electron-microscopically examining substances of weakelectron-optical contrasts, which comprises adding to the slib stance tobe examined a colloid of noble metal so as to obtain a conglomerate ofstrong electron-optical contrasts containing said substance andcolloidal gold particles in juxtaposition and non-reactive as regardseach other, subjecting said conglomerate to electron-microscopicalmagnification, and adjusting the magnification by focusing the magnifiedimage on the contrast.

of the conglomerate.

5. The method of electrran-microscopically examining substances of weakelectron-optical I contrasts, which comprises adding to the substance tobe examined a colloidal metallic oxide non-reactive as regards saidsubstance so as to obtain a conglomerate of strong electron-opticalcontrasts containing in juxtaposition particles of said substance andcolloidal oxide particles, subjecting said conglomerate toelectron-microscopical magnification, and adjusting the magnification byfocusing the magnified image on the contrasts of the conglomerate.

HELMUT RUSKA.

